1. Field of the Invention
The present invention relates generally to frequency modulating circuits and more particularly is directed to a frequency modulating circuit suitable for being used to frequency-modulate a carrier signal with an audio signal, a video signal or the like.
2. Description of the Prior Art
An angle modulating circuit may be used as, for example, a frequency-modulating circuit for audio signals in a magnetic recording and reproducing apparatus or the like in which an audio signal, for example, is angle-modulated on a carrier signal, then recorded together with a video signal by a rotary magnetic head and then reproduced. It is desired that such frequency modulating circuit be used which has an output with a low distortion, reduced high frequency components and a large frequency deviation and which is suitable for being formed as an IC (integrated circuit).
As the frequency modulating circuit for frequency-modulating audio signals used in such magnetic recording and reproducing apparatus, there has been employed a frequency modulating circuit which is formed of oscillatory elements such as L, C, R (inductance coil, capacitor and resistor) and of an amplifier. Since this frequency modulating circuit produces an output signal with a sinusoidal waveform, it provides the advantage that the frequency-modulated signal therefrom is low in distortion. On the other hand, since the center frequency thereof is varied by a variable capacitor and also the frequency vs. capacitance change characteristic of the variable capacitor is non-linear, there is a defect in that it is impossible to increase the frequency deviation width relative to the center frequency. In addition, the degree of modulation thereof can not be raised.
On the other hand, a frequency modulating circuit employing a multivibrator carries out the frequency modulation by using the charging and discharging of its capacitor so that the modulated frequency-frequency deviation characteristic becoms linear, and thus the frequency deviation width is increased. However, since the output from this frequency modulating circuit is of a square waveform, it contains much higher harmonic components. As a result, the FM occupied band width is wide. In addition, since it is necessary for such frequency modulating circuit to use a capacitor of relatively large capacitance, the construction of the above frequency modulating circuit is not suitable for being formed as an IC.